1. Field of the Invention
The present invention relates to the manufacture of solar photo-voltaic cells and an excimer laser generating device for producing a pulsed output at a high repetition rate and with uniform power output across the beam aperture, for use in annealing solar photo-voltaic cells.
2. Description of the Prior Art
In the manufacture of solar photo-voltaic cells, a semiconductor junction must be produced which will respond to incident solar radiation by producing a flow of electrons, and a counterflow of "holes". The element silicon, when suitably doped to form a P-N junction, responds in this fashion. Typical dopant materials include boron, phosphorous and arsenic. The electrical conversion efficiency of conventional solar voltaic cells is from about 10 to 16 percent. That is, 10 to 16 percent of incident radiant solar energy is converted to electrical energy using doped silicon wafers as solar voltaic cells. Other photosensitive semiconductor substrate materials may be substituted for silicon, for example gallium arsenide, germanium, gallium phosphide, indium phosphide cadmium telluride, aluminum antimonide, cadmium sulphide, copper oxide, and others.
The principal problem that arises in the manufacture of solar voltaic cells according to the present state of the art is that in order to implant the dopant material, the underlying silicon crystalline structure is damaged during the ion implantation process. That is, the implantation process removes atoms from an orderly crystalline latticework at the implantation site, and creates partial and disoriented latticework regions. This damage is repaired, according to the current state of the art, by extended heating of the doped silicon wafers following implantation of the dopeant material for several hours at temperatures typically above 200.degree. C. Furthermore, with the present state of the technology, only monocrystalline silicon is useful in the manufacture of solar voltaic cells, since a single crystalline lattice work is necessary in order to achieve a directed current flow, and hence a current which can be applied to external circuitry. The production of monocrystalline silicon is far more costly than polycrystalline or amorphous silicon, and much less readily available. As a consequence, the high cost of manufacture has thus far precluded the use of solar voltaic cells as a source of electrical power for any but the most exotic applications. To date, the use of solar voltaic cells as a source of electrical energy has been commercially significant only in supplying power to vehicles and instruments used beyond the earth's atmosphere, and instruments which must necessarily be used in remote, unattended locations.
There are two general type of conventional dopant implantation techniques used in the manufacture of solar voltaic cells. In the ion implantation technique the dopant is a high energy ion beam of a number of kilovolts. The impurity ions of arsenic, phosphorous or boron are rammed into the lattice structure of the silicon wafer with this high energy beam. This damages the crystal lattice structure of the monocrystalline silicon wafer, which necessitates subsequent annealing. In the other commercially signficiant technique of dopant implantation, the dopant material is applied to the surface of a monocrystalline silicon wafer and thereafter thermally diffuses into the wafer. The thermal diffusion process complicates the manufacture of the cells, and adds to the manufacturing expense the same as high temperature thermal annealing.
Various attempts have been made to utilize laser beams for the purpose of annealing doped silicon wafers following implantation of the dopant material. This annealing has been attempted both to reform monocrystalline silicon structure, and also to transform amorphous or polycrystalline silicon into a monocrystalline structure following dopant ion implantation. For example, U.S. Pat. No. 4,151,008 describes a pulsed laser annealing process which utilizes a neodynium-yag laser beam to effectuate annealing. Such a laser produces a beam which does not directly produce the ultraviolet wavelength so readily absorbed by silicon. Also, the pulse repetition rate achieved with such a laser is relatively low (0-20 pulses per second) limiting the average laser output power, and hence limiting the procesing capability of this type of system. Because of the low repetition rate and infrared wavelength of the yag laser, the annealing process using such a device requires an excessively large input power applied over a prolonged period of time. To achieve an ultraviolet output, an ultraviolet flash lamp was employed in place of the laser. However, the light output produced by the ultraviolet flash lamp is quite difficult to focus in order to achieve the required energy density to effectuate semiconductor annealing. In such an ultraviolet flash lamp system, the energy per pulse achieved in the beam and beam uniformity are both poor. While interesting as a laboratory tool, such a system is not feasible for use in the mass commercial manufacture of solar voltaic cells.
U.S. Pat. No. 4,154,625 also deals with the use of a laser in annealing semiconductor devices, and the fabrication of polycrystalline solar cells in particular. This patent suggests lasers in the optical range, but a ruby laser was utilized. The wavelength of a ruby laser is primarily in the red visible and infrared regions, not the ultraviolet where silicon absorbs energy well. Lasers such as this which project beams in the infrared and visible range are capable of high peak power, but their beam uniformity has historically been poor. The energy level obtained with the ruby laser is quite large, but with the poor beam uniformity achieved large localized temperature variations in the silicon exist. The silicon therefore is melted and reforms to a polycrystalline or monocrystalline structure at some implantation sites while the energy applied is inadequate to induce epitaxial regrowth at adjacent sites. U.S. Pat. No. 4,147,563 utilizes a similar ruby laser to implant impurities in silicon in the manufacture of solar cells, rather than to anneal the silicon following implantation.
U.S. Pat. No. 4,059,461 suggests the use of a continuous wave Nd:YAG laser for purposes of annealing silicon in the fabrication of solar voltaic cells. The laser suggested operates at only 6 or 7 watts of power, however, although lasers of larger power output, such as CO or CO.sub.2 having a power output of 100 watts are contemplated. However, such lasers operate primarily in the infrared region. Because of the lower absorption of silicon in this region, a higher average power is necessary to achieve annealing. This excessive power produces excessive heating of the entire silicon wafer, which can distort or damage the entire wafer. Laser scanning is applied to polycrystalline semiconductor material in annealing. However, the laser systems employed according to this patent do not provide sufficient power and do not obtain an acceptable throughput rate. A complete heating and cooling cycle of doped silicon took approximately 10 minutes. In continuous wave scanning the beam must be focused to a very tiny point. This necessitates intricate mechanical apparatus to control a scan pattern. Also, in continuous wave annealing epitaxial regrowth is induced in the solid phase, not the liquid phase as in pulsed annealing processes. Consequently, the quality of repair to the damaged lattice sites is not as good in continuous wave annealing as contrasted with pulsed annealing. Accordingly, such a device could not feasibly be scaled for the commercial manufacture of solar voltaic cells.